[Researches on the effect of gas humidity on photoacoustic spectroscopy gas detection system].

Environmental influence is the important factor in a photoacoustic spectroscopy gas detection (PASGD) system when it is applied in the industrial field. The experiments show that the sensitivity of condenser microphone is affected mostly by the humidity of the gas under test, leading to the PASGD's result drift. The present paper puts forward a method to eliminate the influence of gas humidity. A speaker is fixed in the photoacoustic cell, whose amplitude is regarded as the sensitivity self-adaption characterization of sound sensor, used to correct the photoacoustic signal amplitude. Thus the problem of sensitivity changing in sound detection with condenser microphone is solved. Based on this method, a photoacoustic experimental setup, equipped with a diode laser, a resonant photoacoustic cell and a lock-in amplifier, was applied to compare the test for different humidity samples. The results show that this method is useful to eliminating the gas humidity influence and enhancing environmental adaptability of PASGD system.

[Theoretical study on radial resonance coupling of cylindrical photoacoustic cells].

Photoacoustic detection of trace concentrations of gases is one of the most sensitive techniques of infrared absorption spectroscopy. High-sensitivity photoacoustic detectors apply an acoustic resonator for the amplification of the weak photoacoustic signal. If the modulation frequency coincides with one of the resonance frequencies of the chamber, a standing acoustic wave is excited and the system works as an acoustic amplifier. The amplification of the resonator relies on the acting mode, quality factor, nature of microphone, and the coupling between electromagnetic radiation and the stand wave resonance mode. With different incidence orientation of the modulated IR laser relative to acoustic chamber, the sound pressure magnitude of resonance mode varies. The influence of different laser incidence orientation on the coupling coefficients of radial resonance mode of cylindrical photoacoustic cells was investigated by both theoretical deduction and numerical computation method. It is concluded that the coupling coefficients have two zeros and two maximums when the laser incidence angle varies from 0 to pi/2. When the incidence angle is 0 or tan(-1) (0.859 2 X 2R/L), the coupling coefficients are zeros and the radial resonance is invalid. When the incidence angle is tan(-1) (0.556 8 X 2R/L) or tan(-1) (2R/L), the coupling coefficients are the maximums and the radial resonance is the strongest. Here R is the radius and L the length of the cell. The results therein before give some theoretical guidelines for photoacoustic cell designing, optimizing, installing and adjusting, and for improvement of detection sensitivity in trace gas detectors through maximal excitement of radial modes in cylindrical acoustic cells.

[Detonation temperature measurement of epoxypropane using instantaneous spectrum method].

After solving the problems of synchronization of the measuring system and the avoidance of false trigger signal, the instantaneous emission spectrum of epoxypropane with an exposure time of 2 micros and a resolution of 0.2 nm was acquired from a side window of a shock tube at the very moment when the epoxypropane transformed from deflagration to detonation. The measuring system consists of an advanced intensified charge-coupled-device spectroscopic detector, a digital delay generator DG535, an explosion shock tube and optical fibers. The DDT process was monitored by pressure transducers. After correcting the intensity of the spectrum obtained, the background curve of the heat radiation intensity of the detonation was given immediately. The detonation temperature of 2 416 K for epoxypropane was derived from fitting the curve with Planck blackbody formula by least squares principle. The detonation temperature of epoxypropane can provide an experimental datum for analyzing the microscopic mechanism of DDT process.

[Using instantaneous spectra to determine dominant species in the DDT process of epoxypropane].

After solving problems of weak light detection, the calibration of the spectral sensitivity of the measuring system, and the synchronization of the measuring system, instantaneous emission spectra of epoxypropane in the process of deflagration to detonation transition (DDT) with the exposure time of 2-8 micros and the resolution of 0. 2 nm were acquired from six different side windows of an explosion shock tube. Using the corrected spectral data, curves of the optical radiant intensity of main reaction products versus the DDT distance from the ignition point were obtained. These curves provided information about the evolution of the reaction and the products during the DDT process. Results indicate that the chemical reaction rate of the gaseous fuel and the corresponding concentrations of intermediate products increased gradually at the deflagration stage, but at the moment of deflagration to detonation transition, the reaction rate increased rapidly and the concentrations! of products increased sharply. Among these main products, concentration increments of molecule CO, and radicals CHO and OH were greater than other products, which means that CO, CHO and OH are the dominant species that affect the DDT process greatly.

[Determination of reaction products of epoxypropane in the process of deflagration to detonation transition by emission spectroscopy].

After solving problems of the synchronization of the measuring system, the detection of weak light, and the avoidance of false trigger signal, the instantaneous emission spectra of epoxypropane in the process of deflagration to detonation transition (DDT) with the exposure time of 1-16 micros and the resolution of 0.2 nm were measured. The spectra were acquired from side windows of an explosion shock tube 0.1 m in inner diameter and 4.0 m in length. The measuring system is made up of an intensified spectroscopic detector ICCD, a SpectraPro-275 spectrograph, and a digital delay generator DG535. By analyzing the spectra obtained, the reaction products OH, CH, C2, C3, CO, CO2, CHO and CH2O were identified according to their characteristic electronic and vibrational bands, which indicates that these molecules and radicals were produced during the DDT process of epoxypropane. The determination of reaction products can provide experimental basis for analyzing and understanding the microscopic mechanism of DDT process.

[Instantaneous emission spectra of epoxypropane in the process of deflagration to detonation transition].

Using an intensified CCD spectroscopic detector (Princeton Instruments, ICCD PI-Max 1024 RB) which can be gated in as little as 5 ns, the synchronization of the measuring system was controlled by a digital delay generator (Stanford Research Systems, DG535), the DG535 was triggered externally by a lab-made electrical pulse generator which transformed the optical trigger signal to an electrical signal, and the light signal from the end window of an explosion shock tube was delivered by an 1 mm in diameter plastic optical fiber to the entrance slit of the spectrometer (grating of 150 g x mm(-1) , central wavelength of 550 nm). The spectrum measurement of the epoxypropane in the process of deflagration to detonation transition (DDT) was then made. The instantaneous emission spectra of epoxypropane at different time of the DDT process with an exposure time of several microseconds were acquired. Results show that at the beginning of the DDT process, the emitted light was very weak and the line spectra of atoms were observed mainly; in the middle process of the DDT, the emitted light became strong and the spectra observed consisted of line spectra of atoms, band spectra of molecules plus continuous spectrum of the thermal radiation; when the detonation was formed, the emitted light got very strong, and the spectra acquired consisted of both line spectra of atoms and band spectra of molecules superimposed on the strong continuum of the thermal radiation.